1. Field of the Invention
This invention relates to nuclear reactors, and specifically to a rodded system for rapid shutdown of the reactor.
2. Description of the Prior Art
A main requirement for most nuclear power facilities is that diverse means for shutting down the reactor exist. Of major import is that diversity exists between the base control system and the emergency shutdown system. A prime concern that the diversity alleviates is common mode failures, as opposed to random independent failures against which redundancy is the means of protection. The major sources of common mode failures are (1) external environment (floods, fires), (2) design or manufacturing deficiencies (improper manufacture of the same part of a system), (3) operating and maintenance errors (incorrect calibration or instruction), and (4) functional deficiencies (unrecognized deficiencies, unanticipated operation changes). Diversity as a main defense against common mode failures may be categorized as (1) functional diversity (instrumentation response to different parameters), (2) equipment diversity (different instruments and mechanisms), (3) location diversity (physical separation), and (4) interfacing diversity (components not affected by failures in interfacing systems). This invention was prompted by the desire to have a diverse shutdown system for the Fast Flux Test Facility (FFTF), a facility which is a major testing ground for sodium cooled fast breeder reactor technology.
This invention will therefore be primarily applicable to liquid metal cooled nuclear reactors, although it can be applied to other reactor types. Most present day water cooled reactors utilize an injection of liquid boric acid solution for emergency shutdown. The properties of boron are such that it has a high cross section for thermal and also fast neutrons, and has no adverse affect on the nuclear system in proper solution. However, it cannot easily be utilized in a sodium cooled reactor due to the resulting exothermic reaction with sodium, and also because of the purification of the liquid sodium coolant that would be required subsequent to testing or emergency actuation of, a boron injecting system. A system which will rapidly admit boron or other high cross section substances to the core region in an encased fashion is therefore typically used. Most typical are control rod drive mechanisms (CRDM), which provide a means to lower an assembly of rods containing a form of boron, or other neutron absorbing materials, into a reactor core. Many existing devices are based upon holding a control rod assembly in an upper position through use of an electromagnet and redundant latch mechanisms. These devices are typically raised to their uppermost position by mechanical latching means. Typical prior art apparatus also include top mounted rack and pinion and drum and cable devices, as well as bottom or top mounted hydraulic piston devices. Some devices assist the downward drop with springs or pistons, since a rapid drop into the core region is required. Some types utilize hydraulic means to assist the downward motion and may also use hydraulic means to raise the control rod assembly from the core. Most prior art shutdown systems also include damping means to break the downward motion and prevent significant damage to the fuel assembly, as does this invention. This invention also incorporates a control rod assembly held in an upper position by electromagnetic means, in one embodiment, and a gravity forced fall. In another embodiment, pneumatic means hold the control rod assembly in the uppermost position. In both instances, pneumatic means are utilized to raise the assembly from the core. Adequate diversity is provided on the FFTF reactor because the shutdown system responds to a variety of plant signals, is of a totally distinct design compared to existing rod drive mechanisms, and will not be affected by assumed failures in the base control system control rod drive mechanisms. The base control system of the FFTF utilizes a control rod drive mechanism of the roller nut type. In that design a stator an drive motor assembly drives the threaded roller nuts which engage a lead screw ultimately connected to the boron type absorber assembly. The roller nuts are disengaged to shutdown the reactor. This diversity is applicable to other reactors as well. Redundancy is provided in the invention disclosed and described herein by means associated with the pneumatic source. This system also provides, as do most prior art devices, a fail-safe design in the sense that loss of either a pneumatic or electromagnetic holding source directly causes the absorber assembly to drop, by gravity, into the reactor core.